1. Field of the Invention
The present invention relates, generally, to methods for fabricating precision dies and, more especially, to methods for chemical milling an etchable workpiece to form a precision stamping or trimming die for fabricating circuit boards where the raised die elements are formed in a highly compressed pattern separated by cavities having a good working depth. The present invention also relates to fine-line dies formed in accordance with these methods.
2. Description of the Background Art
The present inventor's U.S. Pat. Nos. 3,758,350, 4,053,348, and 4,l02,735 disclose, inter alia, methods for making precision stamping dies useful in the manufacture of circuit boards, namely printed wiring boards. Briefly stated, the dies disclosed in the those patents include a plurality of raised die elements separated by cavities, which dies are then used to stamp and adhere a metal foil to a prepared substrate bearing a suitable thermoadhesive. The metal foil is pressed against the substrate and severed by the action of the die elements, which are preferably heated, providing an intimate bond between the metal foil within the pattern of the die elements and the substrate while activating the adhesive.
It is important that the die used in the stamping of such circuit boards be formed with precise and sharp die element edges. If the elements are imperfectly formed there is a tendency for an imperfect stamping operation resulting from the lack of a clean cut; e.g., due to dulled edges which can leave behind small ribbons of foil bridging circuit elements to form an undesired connection. In this regard, it is also important that the raised die elements be separated by cavities having a good working depth to insure adequate penetration of the die elements through the foil.
The trend in manufacturing circuit boards is toward miniaturization without a loss of the ability of the board to carry adequate current. Thus, on the one hand designers strive to provide circuit boards with elements closer and closer together while, on the other hand, the demand for improved current ratings requires thicker and thicker foils be used. These two desires on the part of designers are somewhat antagonistic and have taxed conventional chemical milling techniques to a realistic limit. In manufacturing a fine-line knife-edge foil cutting die (i.e., one having a line and space pattern of approximately 0.015" or less), it is difficult to achieve sufficient cavity depth between patterns with conventional chemical milling processes and thereby achieve the goals mentioned above. This results in part because of inherent limitations on photo resist films used to mask portions of the die block during the milling procedure used to form the cavity.
The typical dry film photo resist materials used to mask portions of the face of a die blank, to render those portions etchant passive, are usually films having a thickness on the order of 0.001-0.002". They are typically made from thermoplastic compositions so that the film may be adhered to the metal die blank by the application of heat and pressure. These combined characteristics of heat sensitivity and relatively thin film thickness have proved to be limiting factors in the adaptability of conventional dry film resists in the formation of fine-line dies. For example, when etching a die blank bearing such a thin-film resist, a process typically conducted by disposing the blank in a bath of ferric chloride etchant heated to about 100.degree. F., etchant begins removing material in the etchant-active regions on the die blank (defined between etchant-passive regions bearing photo resist film) and undercuts the photo resist film at the same time. Optimally, the etchant would simply preferentially attack the region bounded by adjacent films; but this is not even a theoretical possibility given the current state of the art. Rather, as the resist film is undercut, the portion extending over the undercut softens due to the temperature of the etchant and flexes due to the activity within the bath. Thus, a weakened line is developed along the interface between the bonded film and the metal substrate supporting same. Softening and flexing allow the formation of a small radius at this bond line, usually one on the order of 0.001-0.002", encouraging increased undercutting and the formation of an undesirably more positive sidewall slope in the cavity being formed. Providing thicker resist films or films made from compositions which are less sensitive to heat are not realistic approaches to the resolution of this significant problem. This is particularly true when one is faced with the task of etching a fine-line die where line and space patterns are less than about 0.015" and where tight pattern areas may have only a few thousands of an inch separation.
While the optimum limitation on open space in a 0.002" dry film resist is about 0.006", there have been attempts to utilize a pattern where spacing is on the order of 0.004" between patterns. However, this requires a labor-intensive and extremely tedious subsequent processing to remove resist "scum" in the tighter pattern areas while the subsequent etching of such patterns produce only about one-half the cavity depth of more open areas. Other efforts have included the use of a 0.001" resist film, but these too have met with limited acceptance; particularly since these thinner resist films undercut faster and provide less cavity depth overall. Further problems are caused because attempts to trim a dry film resist usually result in chipping and a disruption of the bond between resist and metal at the pattern edges due to the brittle nature of the film. This, in turn, leads to ragged chem-milled edges which are highly undesirable.
Consequently, the need exists to provide a method which will allow the fabrication of a die of the character aforesaid where the pattern of die elements is considerably compressed while the surface profile thereof is maintained with a good edge-to-cavity depth relationship.